rpmsg: Introduce GLINK SPSS transport

GLINK SPSS communicates to remote procs over SMEM but allocates the pipe
descriptors and fifos with a single smem item. The location of this smem
region is shared to the remote proc through two location registers.
After the initial setup, the GLINK SPSS transport behaves the same as
the GLINK SMEM transport.

Change-Id: Ic6d406416f865fa8a01052ab73c4aafd7de773df
Signed-off-by: Chris Lew <clew@codeaurora.org>